Bring a large pot of salted water to a boil. Add the pasta and cook until al dente. Drain and toss with 1 tbsp of the olive oil. Set aside until ready to use.
Meanwhile, make the sauce. Melt the butter in a small pan over medium heat. Stir and cook the garlic for 1 minute. Whisk in the whipping cream, milk and Parmesan cheese. Season with salt and pepper, to taste. Allow the sauce to simmer over very low heat until ready to use. Do not allow the sauce to come to a boil.
Meanwhile, combine the breadcrumb with the 1 tbsp of the Cajun seasoning in a shallow bowl (I use a resealable bag). Coat the chicken evenly in the breadcrumb mixture.
Heat the remaining 3 tbsp of oil in a large skillet over medium-high heat. Add the chicken and cook on both sides until the chicken is browned and cooked through, about 5 minutes per side. Drain the chicken on paper towels and set aside.
Pour the Alfredo sauce over the cooked pasta and stir in the cooked chicken. Stir in the mozzarella cheese, the remaining 1 tbsp of the Cajun seasoning (or to taste -- start with 1 tsp at a time if you're unsure) and onion salt. Season with salt and pepper, to taste.